During setup, your will be asked to install the mailvelope browser extension. Mailvelope is a company specialized in end-to-end encryption and has partnered with GMX to ensure the integrity of Encrypted Communication. GMX guarantees that this is a trustworthy program. Mailvelope will carry out any tasks that need to take place locally on your device (as opposed to on a GMX server):
Why? Real end-to-end encryption will only work, if the operations actually take place on your end of the line, and not within the GMX server infrastructure.
All mailvelope use a characteristic background tiled with lock icons. You can customize size, color and angle of these lock icons to make sure you're not being fooled by a malicious website claiming to be mailvelope. See mailvelope.com/help for a more detailed documentation.
The binaries provided by GMX are signed and can be checked for integrity by any PGP tool. Please find our code signing key at https://img.ui-portal.de/hilfe/codesigning.mailvelope.com.pub.asc. Its fingerprint is 16F4 5CCC 1848 12B4 AC93 0EF3 60DD 1F16 B339 0CD1.
Confused? You don't need to understand this section to use mailvelope. The point is that any software developer can verify that mailvelope works as described and doesn't include backdoors or other shenanigans.